Check if tok exists before use

The problem is described in #8078 and was introduced in 151759b.

I am not super happy with the solution here. It seems that checks for
required `load` keys are done differently all over master.py. The reason
why I choose to check for `tok` separately was to be able to pinpoint
which minions have not been upgraded to 0.17.1.

Moving forward the checks for `id`, `data`, `tgt`, `fun`, etc could be
logged the same way, if appropriate.